Electro-acoustic transducers, and in particular electro-dynamic transducers, are widely used in e.g. telecommunication equipment such as wired, mobile or cellular telephones, and hearing aids where small size is a requirement. Both transducers used as microphones and loudspeakers (speakers) are used for transforming acoustical signals into electrical signals and vice versa. For both microphones and speakers to be used in miniature equipment it is essential that the transducer is tolerant with respect to vibrations in order to avoid unintended noise or feedback problems.
With a microphone and speaker placed close to each other, such as especially in hearing aids, a feedback loop may occur due to vibrations created by the speaker. The vibrations from the speaker are transmitted via the housing to the microphone which, to a certain degree, will convert the vibration to an electrical signal being amplified and again converted to sound and vibrations via the speaker, thus creating an unintended feedback loop. Such loops may lead to reduced sound quality. In case of large gains, such as in hearing aids, the mentioned feedback loops may even lead to disturbing howling sounds, and thus being a limiting factor in the maximum possible gain of the hearing aid.
In prior art, a rubber boot construction or a box-in-a-box construction has been used to establish vibration isolation. In the box-in-a-box construction the receiver was mounted in a very compliant gasket to obtain the necessary vibration isolation. The extra housing can also be used as magnetically shielding when a hearing aid including a tele-coil is used.
Another way of providing vibration isolation is to apply two identical receivers (dual receiver) coupled back-to-back and thereby reduce vibrations from the overall system.
A disadvantage of the rubber boot construction is that it does not provide enough vibration isolation and it is very difficult to design and control the design parameters. The box-in-a-box construction easily gets very large and bulky.
The dual receiver construction does not completely cancel out vibrations because such a receiver always generate rotational components when using balanced armature receivers, such as used within hearing aids. When using a dual receiver or a speaker with two traditional radial electro-dynamic transducers, a larger degree of vibration isolation can be obtained. Furthermore, for mobile phones such a transducer may be used as vibration generator, for generating a silent alarm signal, thus, saving weight and space for a separate vibration generator.
JP 11 308691 A (abstract in english) describes a speaker system comprising two diaphragms and one common magnetic circuit. As the two diaphragms move in opposite directions, the vibration force provided to the magnetic circuit is minimised, and the electroacoustic efficiency is increased compared to one diaphragm speakers. However, the speaker system described in JP 11 308691 A has a disadvantage since its magnetic circuit and the diaphragms are circular with the entire coil positioned in the circular magnetic gap. Therefore, in order for such a speaker to provide low distortion the design is critical with respect to production tolerances, such as centering of the voice coil. In addition, the magnetic circuit is bulky and is thus not suited for applications with very limited space available, especially with respect to the height of the speaker system. Furthermore, the speaker system of JP 11 308691 A requires a large number of single components, and therefore it is not suited for low cost mass production.
JP 07 131893 A (abstract in english) describes a two diaphragm speaker with one common magnetic circuit. The speaker described in JP 07 131893 A aims at radiating highly bidirectional sound without phase deviation. This is obtained by integrating two oscillating systems into one body. The diaphragms and magnetic circuit are circular, and the entire circular voice coil is positioned in the magnetic gap. The design has a number of disadvantages. The design is bulky by nature, since it relates to the art of superior bidirectional speakers where parameters such as size and weight is not important. The speaker of JP 07 1131893 A is therefore not suited for miniature design. In addition, the above mentioned problem concerning distortion caused by non perfect symmetry in the magnetic circuit is not solved. Furthermore, due to the large number of single components and complicated geometry the design is not suited for low cost mass production.
Thus, there is a need for an electro-acoustic transducer with two diaphragms being substantially vibration insensitive, in case it is used as a microphone, and substantially vibration free in case it is used as a speaker. The transducer must be suited for miniature applications such as telecommunication equipment and hearing aids.